A conventional method for electrophotographic type image forming apparatuses, such as digital copying machines and laser printers, effects tone by changing the size of basic pixels (one dot). However with such an electrophotographic method which employs photoreceptor bodies, environmental factors such as humidity causes changes in properties of, for example, the photoreceptor body, the developer agent for developing electrostatic images formed on the photoreceptor body by exposure, and the electric charger device for charging the photoreceptor body. The pixels consequently varies in size even under the same image forming conditions, posing a problem in effecting good tone display.
So "image stabilizer devices" have been developed to stabilize images by preventing deterioration thereof due to aging and environmental changes such as humidity and temperature through the control of image forming conditions for the image forming section, such as charging, exposure, and developing. Such a control of image forming conditions for the image forming section to stabilize images will be referred hereinafter to as a process control.
For instance, a toner image is formed for test purposes under predetermined conditions on a photoreceptor body or on a transfer medium, such as a transfer body or intermediate transfer body, provided as a photoreceptor body. The amount of light reflected at an area of less than ten square centimeters is then measured by a reflection type optical sensor. Finally, a process control is conducted based on the differences between the measured value and the reference value. This method is embodied for example with two toner images of different densities, i.e. bright and dark, and two reflection type sensors for measuring the densities thereof.
However, with reflection type sensor reading merely average densities, the method falls short of telling whether the difference between the measured value and the reference value is caused by a change of a pixel in size or an increase or decrease in the amount of adhering toner per unit area. The accompanying result is an unsatisfactory process control and ineffectual image stabilization.
To solve such problems, Tokukaisho 63-280275 (Japanese Laid-Open Patent Application No. 63-280275/1987) discloses a process control being conducted by means of a linear toner image formed on a transfer belt so as to extend in the principal scanning direction (corresponding to the scanning direction of the laser beam, which is perpendicular to the direction of the rotation of the transfer belt) and a reflection type sensor reading the size of the toner image in the auxiliary scanning direction (perpendicular to the principal scanning direction), i.e. the line width.
Another method actively in commercial use is to employ a CCD (Charge Coupled Device) line sensor for reading, for example, the position of the toner image formed on the transfer belt.
As mentioned above, in digital copying machines and other devices, an electrostatic image is formed by the scanning with laser light, and tone display is effected by pulse-modulating the image data and thus controlling the ON time in a single pixel. As a result, the formed pixel varies in size in the principal scanning direction depending on tone, whereas hardly in the auxiliary scanning direction. The size in the auxiliary scanning direction is determined by the beam diameter of the laser light.
Therefore, the process control based on the measurement of the size of an elemental pixel in the auxiliary scanning direction as disclosed in Tokukaisho 63-280275 above does not always offer satisfactory levels of control.
In addition, Tokukaisho 63-280275 discloses measurement, using reflection type sensor, of the line width of the toner image formed on the transfer belt, but it does not elaborate on the configuration of the reflection type sensor. A typical reflection type sensor covers an area of not less than a few square centimeters at a single reading, and it is highly unlikely that such a sensor is capable of detecting the size of the pixels for a digital copying machine or printer having a resolution of a few hundred dpi.
On the other hand, in a configuration where a CCD line sensor is used to read the pattern, it is possible to detect the size of the pixels for a digital copying machine and printer having a resolution of a few hundred dpi. Nevertheless, CCD line sensors allow only limited freedom in spatial arrangement due to their relatively large sizes, and are more costly than reflection type sensors and other devices. In addition, the CCD line sensor includes a plurality of integrated photoreceptor elements and therefore requires compensation for variations between those photoreceptor elements, resulting in a complex drive circuit and increased costs.